In recent years, demand for rechargeable lithium ion secondary batteries as power sources of portable electronic devices for digital communication, such as cell phones, personal digital assistants (PDAs) and notebooks, portable electronic devices, such as digital cameras, camcorders and MP3 players, and electric vehicles, has increased exponentially. The performance of these devices is particularly influenced by secondary batteries as important components of the devices. Accordingly, there is a strong need for high-performance batteries.
With the recent rapid development of electronic device technologies, device products have become increasingly compact and lightweight and their driving voltage has been lowered. Most commercially available lithium secondary batteries are limited to 4V class. Under these circumstances, there is an urgent need for the development of 3V class batteries necessary to drive electronic devices at low voltages.
On the other hand, cathode active materials compose the largest portion of the price of materials for lithium secondary batteries. Cathode active materials are most generally prepared by a solid-state reaction process. According to the solid-state reaction process, cathode active materials are prepared by mixing carbonates and hydroxides of respective constituent elements as starting materials, firing the mixture, and repeating the procedure. Disadvantages of the solid-state reaction process are that solid phase materials are difficult to form into a solid solution, large quantities of impurities are introduced during mixing, control over the size of particles to a constant level is difficult, and high temperature and long preparation time are involved. In contrast, carbonate coprecipitation, which is a process wherein constituent elements can be controlled to an atomic range, enables preparation of starting materials of cathode active materials for lithium secondary batteries, advantageously leading to preparation of spherical metal complex carbonates.
Current research on the development of spinel oxides has concentrated on LiMxMn(2−x)O4 in which 4V class LiMn2O4 and Mn are partly replaced with a transition metal. The spinel oxide LiMxMn(2−x)O4 can be utilized even in the 5V region. However, the development of cathode active materials for 3V class lithium secondary batteries has a limitation because of the structural transition (Jahn-Teller distortion) due to Mn3+. 3V class cathode active materials for lithium secondary batteries are currently being developed in the form of spinel oxides and layered LixMnO2. However, the layered LixMnO2 disadvantageously undergoes sudden transition into a spinel phase as the charge/discharge cycles are proceeded. Further, Japanese Patent Laid-open Nos. 2001-180937 and 2000-243339 report Li4Mn5O12 as a spinel oxide. However, the amount of oxygen must be controlled in an inert atmosphere or under vacuum to prepare the spinel oxide, and preparation variables are complicated, causing some problems in reproducibility. Further, U.S. Pat. No. 6,361,755B1 reports low-temperature preparation of Li4Mn5O12. This preparation, however, requires a long time ranging from one to five days. Further, U.S. Pat. No. 5,135,732 reports the preparation of LiMn2O4 at a temperature below 400° C. and the electrochemical properties in the potential region of 2.4V˜3.5V. However, batteries using the material show poor cycle characteristics, making it unsuitable for practical use.